Fast rotor repair also improves efficiency

1 August 2019

Cracked steam turbine rotor repaired and reinstalled with 8% improvement in performance. By Claudia Proeger, Sulzer.

When cracks were encountered in a steam turbine rotor at an Indonesian geothermal plant, the plant owner/operator opted for repair rather than replacement. The repair took sixteen weeks, compared with at least one year for a new rotor to be provided by the OEM. The repair also resulted in an 8% efficiency improvement.

The Indonesian geothermal power plant had been experiencing issues with one of its 60 MW steam turbines. During the previous two years, the pre-existing maintenance provider had repeatedly tried to repair the turbine, but a vibration problem persisted. Despite several attempts to low speed balance the rotor at the maintenance provider’s premises, it still exhibited high vibration levels.

During one of these off-site balancing efforts, the plant engineers discovered deep cracks. These appeared to have been due to a previous repair by another service company. As a specialist in the repair of steam turbines, Sulzer was called in to resolve the problem.

Sulzer’s initial inspection revealed that the rotor had cracks on the radius section between the last stage disk and the gland seal area located on both the governor and the generator sides.

In addition, there was considerable evidence of erosion on the blades, the disks and the balance correction holes. 

The initial plan was to machine out the material until the cracks were removed and then rebuild the shaft using submerged arc welding before machining it to nominal dimensions. However, after comprehensive inspection at Sulzer, the crack propagation already had a spiral shape through the centre of the shaft, making it impossible to machine out the crack area only. Sulzer then came up with a repair proposal that involved designing a stub shaft that would be used to join the two pieces together before the shaft was rebuilt to its nominal dimensions.

With the location of the repair being so close to the 6th disk, some additional repairs would be needed to this disk, which would extend the overall time to complete the project. To save time, Sulzer proposed both stage 6 disks would be removed and possibly reinstated at a later date. This was then agreed by the owner/operator.

Joining two sections of a turbine rotor requires considerable expertise, including computer modelling and finite element analysis (FEA) to ensure that the proposed design will withstand the stresses of normal operation. The FEA was also carried out at an overspeed of 3600 rpm to ensure that the centrifugal loading on the disks would not cause any damage to the rotor shaft after the repair. With all the necessary analyses completed, the machine shop started to prepare the two rotor shaft sections for the addition of the stub shaft.

In-house precision machining enabled the stub shaft to be shrink-fitted into the prepared connections before the whole joint area was preheated prior to the welding process. Using precision-controlled, submerged arc welding equipment, the stub shaft was built up to a level that would allow it to be machined back to the required dimensions. Once the original dimensions had been achieved, non-destructive testing was carried out to ensure there were no flaws in the completed rotor assembly.

These processes were repeated to remove the cracks in the thrust end of the rotor as well, bringing the completed assembly back to finished dimensions. Once all the machining was complete, the rotor was dynamically balanced before being shipped back to the plant.

While the repairs were being completed on the rotor, the field service team was working at the plant site to repair the diaphragm and improve the sealing of the casing. This work would be influential in improving the efficiency of the steam turbine.

Prior to the project, the turbine required 393 t/h of steam to produce 53.4 MW. Despite one set of disks being removed, the repair to the static and rotor components of the turbine enabled it to maintain an output of 55.1 MW with 374 t/h of steam, an 8% improvement in efficiency.

When the repaired turbine rotor arrived back on site, the field service team carried out the installation and commissioning,
which included vibration testing at full load.

All the results were well within the original specifications and the generator has remained at full capacity ever since. 

Precision machining was required to ensure a perfect fit
The stub shaft was shrink-fitted to the shaft before being welded in place
Dye penetrant testing was completed to ensure the new rotor had no flaws

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